1. Field of the Invention:
The present invention relates to a distributed constant type inductance and capacitance noise filter which will be referred to "LC noise filter" hereinafter.
2. Description of the Prior Art:
There is known a noise filter which comprises a first conductor 12 spirally formed on an insulative substrate 10 made of ceramic or the like at one side as shown in FIG. 17A of the accompanying drawings and a ground conductor 14 formed on the same substrate at the other side as shown in FIG. 17B.
As shown in FIG. 18, such a noise filter functions as an LC noise filter wherein the first spiral-shaped conductor 12 provides an inductance (L) while an electrostatic distributed-constant capacitance (C) is provided between the first spiral-shaped conductor 12 and the ground conductor 14.
However, such an LC noise filter constructed according to the prior art has the following problems:
(a) First Problem:
The prior art noise filter causes an eddy current to produce in the ground conductor 14 which is located on the substrate 10 at its bottom face. Therefore, the first spiral-shaped conductor 12 could not provide such sufficient inductance as expected. As a result, the prior art noise filter only functions to provide electrical characteristics resembling those of a capacitor, rather than as an LC noise filter.
In other words, the first spiral-shaped conductor 12 of the prior art LC noise filter is coupled capacitively and also inductively with the ground conductor 14. Therefore, the current passing through the first spiral-shaped conductor 12 creates a magnetic flux which in turn creates an electromotive force in the ground conductor 14. Such an electromotive force causes a short-circuit current to be produce in the ground conductor 14 as shown by double-headed circular line A in FIG. 17B.
If it is assumed that the first spiral-shaped conductor 12 functions a primary coil in a transformer, the ground conductor 14 acts as if a secondary coil in the same transformer. For such a reason, the first spiral-shaped conductor 12 cannot provide such sufficient inductance level as expected.
(b) Second Problem:
This noise filter is adapted to eliminate noises included in signals which are provided in the opposite terminals 16 and 18 of the first conductor 12 functioning as an inductor. If the frequency of such signals is increased, a short-circuit will be created between each adjacent line in the first spirally formed conductor 12, as shown by arrow B. As a result, the first conductor 12 will not function as an inductor.
Such a short-circuit is more frequently created as the frequency of the provided signals is increased. Therefore, the conventional noise filters could not be used as high-frequency noise filters.